The invention relates to a circuit arrangement for supplying an operating voltage to a number of connectable and detachable circuit units from an operating voltage source.
Connectable and detachable circuit units include circuits such as subscriber terminal circuits or groups of subscriber terminal circuits as used in telephone switching exchanges. Generally, a common voltage transformer supplies an operating voltage of approximately 5 volts to the circuit units.
In applications involving circuit units there is a need to exchange an older unit with a newer unit or connect additional units without disturbing the operation of the other existing units. Such additional connections are difficult, because the units also contain capacitors. The capacitors need to be charged. Therefore in the first moment of the connection, an increased load of the operating voltage occurs due to charging processes of the capacitors. In situations where the current efficiency of the voltage transformer is limited, the supply lines to the voltage transformers are not generally low-resistance, and a rapid compensation of the current surge cannot be carried out problem free, thus a dip in the operating voltage is expected. At a supply voltage of 5 V, a typical voltage dip lies in the range from 0.3 to 0.7 V, and the time duration lies in the microsecond (.mu.s) range.
Switching circuits used in the subscriber terminal circuits operate securely only in the range from 4.5 to 5.5 V. In this voltage range disturbances in the operation of the circuit units often occurs. A possible solution to this problem is to provide the circuit units with what is called a leading contact pin. A special voltage is applied to the leading contact during the connection and before the actual operating voltage is applied. The special voltage charges the capacitors of the unit in a delayed fashion. The special voltage is derived from the normal operating voltage and is supplied via a centrally provided inductor to the charge pulse.
In the unit itself, a diode decouples the mentioned special voltage from the standard operating voltage. This solution is undesirable because of the high costs associated with the special contacts, the central coil and the decoupling diodes.
If a decoupling diode is defective, i.e. it is short-circuited or interrupted, the operation of the circuit is adversely affected. Even in units that are not sensitive to the voltage dips, but cause such voltage dips, it is not possible to forego the mentioned individual measures if overall a combination of units existing in the system that are sensitive to operating voltage dips.